Patent Literature 1 discloses a light irradiation apparatus that irradiates a micro-plate that holds a measurement target in a plurality of wells with light. This light irradiation apparatus includes a light guide member having a main surface in which a plurality of convex portions are formed, a back surface that is a surface opposite to the main surface, and side surfaces substantially perpendicular to the main surface. Further, this light irradiation apparatus includes a light source that inputs light to the light guide member from the side surface of the light guide member. The light guide member is disposed so that an upper surface of the convex portion is in contact with a back surface of the micro-plate.